1. Field of the Invention
The present invention is broadly concerned with a low cost process for the production of solidified alkaline earth salts from starting liquid brines containing the relevant salts. The method is particularly adapted for the production of magnesium chloride hexahydrate (MgCl.sub.2.6H.sub.2 O, also known as bischofite), although other salts such as calcium chloride, calcium nitrate and magnesium nitrate may also be prepared. The invention contemplates production of salts of this type by feeding a brine into a fluidized bed drying chamber in order to dehydrate the brine using a relatively short time fluidizing step.
2. Description of the Prior Art
Magnesium chloride hexahydrate is used primarily in the production of magnesium metal and as a gauging solution for the production of oxychloride cements for flooring, plaster, fire-resistant panels, fire proofing of steel beams, and grinding wheels; it also has a significant use as a road deicer. Magnesium chloride is also used as a fire proofing agent for wood, as a dust binder on roads and in mines, in sugar-beet processing, textiles, water treatment, and as a fire extinguishing agent. Although magnesium chloride forms hydrates with 2, 4, 6, 8 and 12 molecules of water, the hexahydrate and 12-hydrated form are of primary commercial importance. Magnesium chloride is a main constituent of sea water and is found in most natural brines and evaporite deposits. It occurs in scattered deposits as the mineral bischofite and in large commercial deposits as the mineral carnallite.
A number of methods have been followed in the past for the production of magnesium chloride hexahydrate. One of the most common techniques involves the evaporation of sea water and natural brines, but this has heretofore only been economical where the dilute solutions can be preconcentrated by solar evaporation. In one specific example of this process, brine from the Great Salt Lake containing approximately 35% by weight MgCl.sub.2 solution (nominally MgCl.sub.2 .multidot.12H.sub.2 O) is first subjected to solar evaporation, followed by vacuum evaporation until the 6-hydrated form of MgCl.sub.2 and MgSO.sub.4 are precipitated. Thereafter, the precipitated crystals are heated to 120.degree.-150.degree. C. to redissolve the MgCl.sub.2 .multidot.6H.sub.2 O and allow removal of magnesium sulfate. The MgCl.sub.2.6H.sub.2 O can then be crystallized out. Alternately, the initially evaporated solution can be maintained at 120.degree. C. until the magnesium sulfate content decreases to less than 20 g/L as a result of crystallization of kieserite. The MgCl.sub.2.6H.sub.2 O is then crystallized by vacuum evaporation at 90.degree. C.
The prime difficulty with evaporative processes of this type stems from the high energy input and hence costs associated with the ultimate production of MgCl.sub.2.6H.sub.2 O. Indeed, the cost of brine dehydration is by far the most significant expense associated with production of the desired product.
It has also been known to manufacture magnesium chloride as a by-product of the potash industry or by direct chlorination of magnesium oxide in the presence of an organic reducing agent. However, these methods are generally even more expensive than those involving evaporation of brines.
In a similar fashion, salts such as calcium chloride have been produced in commercial quantities by a number of processes including the refining of natural brines, reaction of calcium hydroxide with ammonium chloride in Solvay soda ash production, and the reaction of hydrochloric acid with calcium carbonate. Here again, the brine processes are relatively expensive, and involve multiple steps such as reaction of the brines with lime and subsequent concentration.
There is therefore a need in the art for a simplified, low cost process for the production of alkaline earth metal salts, and particularly MgCl.sub.2 6H.sub.2 O, which avoids multiple, energy-intensive evaporation and/or chemical separation steps.